The invention relates to wireless communication networks, and, more particularly, to techniques for effective wireless communication in the presence of fading and other degradations.
The physical limitations of a wireless channel pose significant challenges for reliable communication. A variety of techniques have been devised to address such issues, including antenna diversity which is seen as a practical and effective technique for reducing the effect of multipath fading in most scattering environments. The classical approach to antenna diversity is to use multiple antennas at the receiver and perform combining or some form of selection to improve the quality of the received signal. Recently, transmitter diversity techniques have been explored, primarily motivated by the feasibility of having multiple antennas at the base station. Spatial multiplexing provided by transmitter diversity facilitates multiple data pipes within the same frequency band, thereby yielding a linear increase in capacity. It has also been discovered that an effective approach to increasing the data rate as well as the power efficiency over wireless channels is to introduce temporal and spatial correlation into signals transmitted from different antennas. This has led to the design of what are referred to in the art as “space-time codes” in which information is transmitted as codewords from multiple antennas at multiple time intervals typically in the form of complex valued amplitudes modulated onto a carrier wave. See, e.g., J.-C. Guey, M. P. Fitz, M. R. Bell, and W.-Y. Kuo, “Signal Design for Transmitter Diversity Wireless Communication Systems over Rayleigh Fading Channels,” Proc. IEEE VTC'96, pp. 136-140, 1996; V. Tarokh, N. Seshadri, A. R. Calderbank, “Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction,” IEEE Trans. Inform. Theory, vol. 44, pp. 744-765, March 1998.
Linear dispersion (LD) codes, for example, are a form of space-time codes that use a linear modulation framework where the transmitted codeword is a linear combination over space and time of certain dispersion matrices with the transmitted symbols. See B. Hassibi and B. Hochwald, “High-Rate Codes that are Linear in Space and Time”, IEEE Trans. Inform. Theory, vol. 48, pp. 1804-1824, July 2002. Linear dispersion codes have the advantages of a very simple encoder design and, furthermore, can be decoded very efficiently either by a polynomial time maximum likelihood decoder, i.e., sphere decoder, or by a suboptimal decoder, e.g., a nulling and cancellation receiver. The linear dispersion codes disclosed by Hassibi et al. were designed to optimize average mutual information; unfortunately, maximizing the average mutual information does not necessarily lead to better performance in terms of error rate. More recently, another scheme based on the linear dispersion code framework called threaded algebraic space-time (TAST) coding has been proposed. See H. E. Gamal, and M. O. Damen, “Universal Space-Time Coding,” IEEE Trans. Inform. Theory, vol. 48, pp. 1097-1119, May 2003. TAST codes are designed based on the threaded layering concept and algebraic number theory, and the design focuses on the worst-case pairwise error probability (PEP). The pairwise error probability, however, may not be the main target for performance evaluation also. The actual dependence of error probability on SNR passes not only through the PEPs but also through the “error coefficients” of the code, i.e., the multiplicity of code word pairs that lead to the same PEP. In general it is not true that the codes optimized with respect to the worst case pairwise error probability will end up with optimum bit or frame error performance.
Accordingly, there is a need for a new approach to the construction of space-time codes that can be optimized to a selected performance metric while still remaining flexible enough to handle different decoder structures.